Benedetto—The strophomenide brachiopod Ahtiella in Gondwana
confirmed the taxic metrics as an adequate proxy for assessing morphological disparity (Erwin, 2007, p. 59). If, as evidence presented here suggests, Ahtiella originated from Monorthis through a series of minor transformations, then the impressive morphological gap among ‘typical’ orthides and strophome- nides was bridged through a brief cladogenetic event. At first, such a transition indicates that there is not a definite dis- continuity between species-level evolution (processes that occur within a species or lead to a newspecies) and the origin of higher taxa (macroevolution). Central to this statement is the assumption that higher taxa are evolutionary entities char- acterized by a significant morphological disparity achieved over a long period of time, then the greater the time elapsed since their origin from a common ancestor, the larger morphological disparity. In the present case study, it would be expected that the highly plesiomorphic basal forms (i.e., ahtiellins) of a given higher taxon (i.e., strophomenides) are more similar to their putative ancestors (i.e., hesperonomiids) than they are to the more derived (apomorphic) end-members of the same clade (i.e., sowerbyellids, aegiromenids). In other words, morpholo- gical discontinuity becomes minimal at a point closer to the initial divergence of two phylogenetically related higher taxa.A consequence of this is that assignation of basal forms to one or another higher taxon can be difficult in the lack of a well- supported phylogeny. The need of a ‘shoehorn’ to classify such earliest members of a given higher taxon into a specified order or suborder has also been noted in other groups of marine benthic organisms. For instance, the clade Bivalvia includes a number of early representatives of Tremadocian-Floian age that lack certain apomorphies defining more derived crown groups. In this respect, it has been suggested that such basal taxa can be classified as plesions, i.e., paraphyletic groups having a number of symplesiomorphic traits but morphologically close to a given higher taxon (Fang and Sánchez, 2012). In the case of bra- chiopods, as Carlson (2016, p. 421) stated, numerous higher taxa had been thought to represent grade-level taxa, i.e., not clades, and our evidence indicates that this could be the case of strophomenides.
Materials and methods
Repository and institutional abbreviations.—CEGH-UNC, Centro de Investigaciones en Ciencias de la Tierra CONICET and Universidad Nacional de Córdoba, Argentina; CORD-PZ, Museo de Paleontologia, Universidad Nacional de Córdoba, Argentina; MGM, Museo Geominero, Madrid, Spain.
Systematic paleontology
The systematic classification follows that of the Treatise on Invertebrate Paleontology (Cocks and Rong, 2000). Following Congreve et al. (2015), the genus Ahtiella is referred to the ‘Plectambonitoidea’ with the quotation marks denoting that the superfamily is paraphyletic (Wiley, 1979).
Order Strophomenida Öpik, 1934
Superfamily ‘Plectambonitoidea’ Jones, 1928 Family Taffiidae Schuchert and Cooper, 1931 Subfamily Ahtiellinae Öpik, 1933
781
Figure 8. Phylogenetic relationships of taxa analyzed calibrated to chronostratigraphic scale. Apomorphies defining numbered nodes are discussed in the text.
Genus Ahtiella Öpik, 1932
Type species.—Ahtiella lirata Öpik, 1932 from the Darriwilian (Middle Ordovician) of Tsitri, Estonia, by original designation.
Diagnosis (emended).—Shell resupinate, variably geniculate; ornamentation subequally multicostellate, ramicostellate or parvicostellate, often with posterolateral rugae. Dental plates widely spaced converging to valve floor, enclosing posteriorly subtriangular to subquadrate ventral muscle field. Interior of ventral valve with thickened margin defining disc and trail. Cardinal process simple. Dorsal median ridge broad, rounded. Dorsal platform variably developed. Saccate mantle canal system usually strongly impressed either along shell margin or whole interior.
Ahtiella argentina Benedetto and Herrera, 1986 Figure 9.1–9.23
1986 Ahtiella argentina Benedetto and Herrera, p. 114, pl. 1, figs. 4−21.
2002 Ahtiella argentina; Benedetto, p. 114, pl. 2, figs. 20, 21. 2003a Ahtiella argentina; Benedetto, p. 201, pl. 9, figs. 9–12. 2009 Ahtiella argentina; Benedetto et al., fig. 9f.
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